Detector having self-calibration function

ABSTRACT

A detector for detecting a physical quantity as a quantity of electricity has a detection portion, a portion for stimulating the detection portion and a signal processing portion, wherein a calibrating signal is supplied from the signal processing portion to the detection portion via the stimulating portion so as to measure a specific response of the detection portion whereby a self-calibration and a correction of the characteristics are performed in accordance with the amount of the change in the response.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a detector for detecting a physicalquantity from a value of an electric signal converted from the physicalquantity, and, more particularly, to a detector having aself-calibration function and a characteristic corrective function.

2. Related Art Statement

Hitherto, a conventional detector has been, as disclosed in, forexample, Japanese Patent Unexamined Publication No. 61-31952, arrangedin such a manner that the measuring operation is stopped so as to starta calibrating operation which is performed as the offline work.Furthermore, there has been disclosed, in Japanese Patent UnexaminationPublication No. 61-212753, an apparatus capable of diagnosingdeterioration by analyzing the characteristics observed in the detector.However, the apparatus of this type also performs, as the offline work,the operation for diagnosing the deterioration.

The conventional calibration has been realized for the purpose ofautomating the offline work. Furthermore, there has been a proposal thatthe reliability of a detector is improved by observing the line andgiving an alarm if necessary as the online work. However, since nomeasure has been taken for performing the calibration as the onlinework, a problem takes place that the measurement is stopped for arelatively long time in comparison with the time in which the value ofthe measurement can be changed.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a detector capable ofperforming a self-calibration as the online work.

Another object of the present invention is to provide a detector systemwhich can subject a significantly wide portion or a multiplicity ofportions to calibration and correction from a remote position and whichcan be easily maintained and safely operated.

A still further object of the present invention is to provide a compactdetector which is provided with self-calibration and correctivefunctions and which can be easily manufactured.

In order to achieve the above-described objects, the calibration or thecorrective operation must be completed in a significantly short time incomparison with the time in which the value of the measurement can bechanged. The reason for this lies in that the data of the measurementmust be protected from a disorder or an error due to the calibration orthe corrective operation performed during the measurement operation. Asfor the device for processing an electric signal, significantly highspeed semiconductor ICs are available recently due to the progress ofthe LSI technology. Therefore, the thus realized speed of processing theelectric signal can cope with the time of several tens to 100 μs whichis the value necessary to conduct measurements in automobile in whichthe values to be measured are varied in a relatively short time.Therefore, unsolved problem is to shorten the time taken to operate thedetection means. Accordingly, the present invention employs stimulatingmeans disposed adjacent to the detection means so as to stimulate andoperate the detection means. A structure can be realized in which asmall sensor or detector, the size of which is, for example, severalhundreds of μm, and an actuator, that is, the stimulating device can beintegrally formed by utilizing the micromachining technology for siliconor the like which has been remarkably progressed recently. Therefore, acompact and integrally formed stimulating device is able to apply acalibration signal, as a stimulation, to the detector without delay.

In order to achieve the other object of the invention, it is necessaryto supply an accurate calibration signal to the detector and tocorrectly measure the response of the detector. Therefore, according toan embodiment of the present invention, a structure is employed in whicha calibration signal is supplied to the detector by a signal processingcircuit including an accurate and high resolution analog-to-digitalconverter. Then, a responding electric signal which is inherent to thedetector is then processed. Furthermore, a suitable self-calibrationalgorithm is accurately and quickly performed by a microcomputer.

In order to achieve the other object of the invention, a structure isemployed according to an embodiment of the present invention, in whichthe processing device is provided with a communication function, theself-calibration and characteristics correction are instructed from aremote position by another communication device and the result of theself-calibration and the characteristic correction are confirmed.

The detector according to the present invention is preferably structuredsuch that the stimulating means is formed adjacent to and integrallywith the detection means so that the calibration signal can be suppliedthrough the stimulating means. Therefore, the delay of response from thedetector can be significantly prevented. Furthermore, a high-speedsignal processing circuit can be employed to shorten the time requiredfor completing the self-calibration in comparison to the time in whichthe values to be measured are changed. Therefore, even if theself-calibration is performed during the measurement operation, theoutput from the detector can be protected from disorder. Therefore, aso-called "online calibration" can be realized.

Furthermore, the characteristics obtained during operation are alwayscorrected in accordance with a comparison made with the initialcharacteristics of the detector based on a calibration and correctivealgorithm previously prepared in the processing means. Therefore, theinitial performance can be maintained to significantly improve thereliability.

Other and further objects, features and advantages of the invention willbe made more apparent by the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the basic structure of an embodiment of the presentinvention;

FIG. 2 is a circuit diagram for a signal processing means;

FIGS. 3 to 6 illustrate the operation of an electrostatic capacity typepressure sensor;

FIGS. 7A and 7B are flow charts of the operation of a microcomputer;

FIGS. 8 to 11 illustrate the operations of semiconductor accelerationsensors;

FIGS. 12 and 13 illustrate another embodiment of the accelerationsensor;

FIG. 14 illustrates the characteristics of an air-fuel ratio sensor withrespect to an excess air factor;

FIG. 15 illustrates the voltage-electric current characteristics of theair-fuel ratio sensor;

FIG. 16 illustrates the output characteristics of the air-fuel ratiosensor;

FIG. 17 illustrates the structure of a sensor having a self-diagnosisfunction; and

FIG. 18 illustrates the self-diagnosis operation.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to FIGS. 1 and 2, the basic structure of an embodiment of thepresent invention will be described. Reference numeral 1 representsdetection means and 2 represents stimulating means disposed adjacent tothe detection means 2 and integrally therewith. Reference numeral 3represents an assembly of the detection means 1 and the stimulatingmeans 2. Reference numeral 4, as shown in FIG. 1, represents a signalprocessing means structured as shown in FIG. 2 and arranged to supply apower supply voltage Ex for operating the detection means 1 and thestimulating means 2 and to process a calibrating signal to be suppliedto the stimulating means 2. Furthermore, the signal processing means 4has a so-called "signal adjustment function" capable ofamplifying/converting a responding output signal from the detectionmeans 1 as shown in FIGS. 1 and 2. In addition, the signal processingmeans 4 has a function of calibrating the input/output and acharacteristic corrective function realized by digital-data processingperformed by a microcomputer 44 as shown in FIG. 2. Reference numeral 5,as shown in FIG. 1 represents a detector including the above-describedelements. Usually, the detector converts an input physical quantity suchas a pressure, a discharge and an acceleration into a digital quantityof a certain number of bits so as to output it. Reference numeral 7, asshown in FIG. 1, represents a communication device capable oftransmitting/receiving a command signal and an output signal to and fromthe signal processing means 4, the communication device 7 further havinga function of displaying the command signal and the output signal.

FIG. 2 illustrates the specific structure of a circuit serving as thesignal processing means 4. In response to a command issued from themicroprocessor 44 having a memory 45, a multiplexer 411 is operated soas to cause the output signal transmitted from the detection means 1 tobe received by an amplifier 412a and an analog-to-digital converter 42.As a result, the output signal thus received is converted into a digitalsignal. In accordance with the value represented by the digital signal,the signal processing means 4 supplies the power supply voltage Ex orsupplies the calibration signal to the stimulating means 2 via anotheramplifier 421b. As a result, an accurate detection signal capable ofcorrecting an error can be obtained.

Then, the principle of a span calibration operation will be describedwith reference to an electrostatic capacity type pressure sensor so asto specifically describe the invention.

As shown in FIG. 3, the electrostatic capacity type pressure sensor isconstituted in such a manner that an electrode plate 2, which can bedisplaced by a differential pressure ΔP, is disposed between electrodeplates 1a and 1b each of which has an area A. The distance between eachpair of the electrode plates is X₀ under no differential pressure.Furthermore, each of the spaces between the electrodes is filled with asubstance having a dielectric constant e.

When the differential pressure ΔP is applied, the intermediate electrodeis, as shown in FIG. 4, displaced by ΔX. Since the displacement issubstantially in proportion to the differential pressure, therelationship expressed by (ΔX=k·ΔP) is held, where symbol k denotes acompliance (the reciprocal of the spring constant).

Assuming that K is varied as the time proceeds, it can be expressed asthe function of time T, that is, k(T). As a result of theabove-described displacement of the intermediate electrode plate 2,there is generated a capacity difference ΔC between capacities C₁ andC₂, where the capacity C₁ is the capacity between the electrodes 1a and2 and the capacity C₂ is the capacity between the electrodes 2 and 1b.

As a result of the arrangement of a circuit shown in FIG. 5, thecapacity difference ΔC due to the differential pressure can be detectedby the following equation: ##EQU1## where symbol E denotes excitationvoltage, e denotes voltage to be detected and ΔC stands for C₂ - C₁ Asan alternative to this, the voltage can be expressed by the loadeddifferential pressure ΔP as follows: ##EQU2##

As can be seen from the above-described equation (2), detected voltagee_(max), that is, the output span under the maximum differentialpressure ΔP_(max) is changed as the time proceeds if the compliance k ischanged as the time proceeds.

The voltage V of the intermediate electrode is applied so as to generatea displacement Δxv due to the electrostatic force, so that the outputvoltage e generated at this time is measured for the purpose ofcalibrating the above-described span change based on the relationshipsthus obtained.

First, calibrating voltage V shown in FIG. 5 is selected in a mannerexpressed by the following Equation (3):

    V=E/2+v                                                    . . . (3)

However, assuming that (v<E/2), the displacement due to the voltage V isof a value expressed by ΔX shown in FIG. 6 and given by the followingEquation 4: ##EQU3##

The output voltage e at this time is of a level given by the followingequation: ##EQU4##

Therefore, since k(T) can be known by varying V, the span can becalibrated.

Then, the procedure will be described.

It is defined that the outputs are e₁ and e₂ when calibrating voltagesV₁ and V₂ are applied. In order to delete from Equation (5) the termwhich depends upon the pressure, the following calculation is performed:##EQU5##

Then, the proportion to Δe with the initial T being 0 is given asfollows: ##EQU6##

By using d thus obtained, calibrating output can be obtained from thefollowing equation:

    e=d·e(where v=0).

FIGS. 7A and 7B illustrate flow charts for a process to be performed inthe microcomputer. FIG. 7A illustrates the main routine in which theordinary measurement work flows along a route 10. In the route 10, thedetection voltage e at (v=0) in Equation (5) is measured so as to obtaincalibration voltage e by multiplying calibration coefficient d. As aresult, the calibration voltage e, which is the final detected value, isobtained. The above-described calibration coefficient d is calculated ina route 20. The Program for the microcomputer is provided with acalibration measuring sub-routine shown in FIG. 7B, in which thedifferential value Δe between the detection voltages e₁ and e₂ iscalculated, the detection voltages e₁ and e₂ being obtained frommeasurements in which the calibrating pulse voltages V₁ and V₂ areapplied.

The calibration includes an initial calibration and an optionalcalibration. The initial calibration is performed at the delivery of theproduct from a manufacturing plant and conducted such that an initialdifferential value (Δe-init) is calculated and stored by the process ofa route 20. The route 20 is performed as the optional calibration inwhich present differential value (Δe-present) is calculated so as toobtain the calibration coefficient d which is the ratio to Δe-init. Thecalibration coefficient d thus obtained is stored so as to update theprevious value.

As described above, the pressure, the sensitivity drift of which iscorrected by the calibration voltage e obtained by multiplying thedetection voltage e by the calibration coefficient d, can be obtained.

FIGS. 8 and 9 respectively illustrate basic structures of semiconductoracceleration sensors of an electrostatic capacity type and of apiezoelectric resistance type, which are two typical types, the sensorsbeing manufactured by a silicone micromachining technology.

The acceleration sensor is for obtaining an acceleration by ameasurement of an inertia force acting on a predetermined mass in thecase where the acceleration exists. Each of the acceleration sensorsshown in FIGS. 8 and 9 is structured such that a load 53 and acantilever 54 for supporting the load 53 are formed on an intermediatesilicone substrate 51 by anisotropic etching. When acceleration α isapplied, inertia force (E₁ =mα) acts on the load (mass m), causing theload (mass m) to be displaced. On the other hand, the cantilever acts asa spring so that it gives to the load a restoring force expressed by (F₂=kx) (where symbol k denotes a spring constant and x denotes the amountof displacement), the restoring force being given in the directionreverse to the direction of the displacement. As a result, the load isdisplaced to the position at which the above-described two forces arebalanced. From the relationship expressed by (F₁ =F₂ x), thedisplacement x is given by:

    x=md/k . . .                                               (I)

Therefore, the acceleration a can be obtained from the displacement x.

The electrostatic capacity type acceleration sensor shown in FIG. 8includes an upper fixed electrode 55a and a lower fixed electrode 55aformed on the surfaces of the upper substrate 52a and the lowersubstrate 52b which face the intermediate silicone substrate 51. Theelectrostatic capacity type acceleration sensor acts to measure theacceleration by obtaining the displacement x of the Equation (I) fromthe electrostatic capacity between the fixed electrodes and the load(movable electrode).

On the other hand, the piezoelectric resistance type acceleration sensorshown in FIG. 9 is structured such that a gauge portion 58 comprising animpurity diffusion region is formed on the cantilever 54. When the load53 is displaced by an acceleration, the cantilever 54 is deformed,causing the electric resistance of the gauge portion 58 to be changed bythe piezoelectric resistance effect. The displacement can be obtainedfrom the electric resistance of the gauge portion so that theacceleration is obtained.

Thus, an output signal V(α) corresponding to the acceleration can beobtained by the signal processing circuit which processes theelectrostatic capacity between the load and the fixed electrode or theelectric resistance of the gauge portion. Since the output and theacceleration α are usually processed so as to keep a linearrelationship, the output V(α) is expressed by the following equation:

    V(α)=pα+q. . .                                 (II)

It is assumed that the acceleration sensor is changed as the timeproceeds for some reason. If the change takes place with the linearrelationship between the acceleration and the output (substantially)maintained, the output becomes the function of the time. Therefore, theoutput becomes as follows:

    V(α, t)=p(t)α+q(t) . . .                       (III)

If the span p(t) and the zero point q(t) of the acceleration-outputcharacteristics (III) have been correctly known, the acceleration α canbe accurately obtained by measuring the output V(α, t).

In the case where p(t) and q(t) are unknown in Equation (III), they canbe obtained by generating two different accelerations α₁ and α₂ by somemethod so as to measure the outputs V(α₁, t) and V(α₂, t) whichcorrespond to the two accelerations α₁ and α₂ Namely, P(t) and q(t) canbe obtained from the following simultaneous equation: ##EQU7##

On the other hand, the acceleration α corresponds to the displacement xof the load in the relationship given by Equation (I). Therefore,determining the acceleration α₁ and α₂ becomes equivalent to determiningthe displacements x₁ and x₂ which correspond to the accelerations α₁ andα₂. Thus, the following relationships are obtained from Equations (I)and (IV): ##EQU8## The predetermined displacements x₁ and x₂ shown inEquation (5) can be relatively easily realized. That is, the structuremay be such that the load is forcibly displaced by an actuator and thecharacteristics of the sensor output V(x, t) sharply varies at thepredetermined certain displacements x₁ and x₂. As an alternative tothis, the structure may be such that any further displacement isinhibited.

FIGS. 10 and 11 illustrate examples of the above-described structures,in which stoppers 60a and 60b are provided for the purpose of preventingany displacement which exceeds a predetermined degree even if anacceleration or an external force acts on the load. If the displacementsx₁ and x₂ obtained when the loads are brought into contact with thestoppers 60a and 60b are previously know, p(t) and q(t) can be obtainedfrom Equations (V) and (VI) by measuring the outputs V(x₁, t) and V(x₂,t) at this time.

In order to displace the load at a desired time so as to bring it intocontact with the stoppers in the case of the electrostatic capacity typesensor, voltage is applied between the load and the upper fixedelectrode 55a or the lower fixed electrode 55b, which acts to obtain theelectrostatic capacity, so as to apply the electrostatic force betweenthem. Also in the case of the piezoelectric resistance type sensor, theupper fixed electrode 55a and the lower fixed electrode 55b are formedand voltage is applied between the load and the upper fixed electrode55a or the lower fixed electrode 55b.

As described above, an advantage can be obtained that the change in theacceleration-output characteristics as the time proceeds can becorrected by a simple calculation from the output obtained byperiodically applying a voltage between the fixed electrodes and theload. Furthermore, the correction can be performed even if theacceleration is being applied to the sensor.

According to the above-described embodiments, the displacement of theload is measured and the acceleration is obtained from the displacementthus measured. A servo type acceleration sensor is also known as atypical acceleration sensor. The servo type acceleration sensor isdesigned such that the displacement of the load due to an accelerationis measured and a signal representing the displacement is fed back.Furthermore, in response to the signal, the load is given, by somemethod, a force in the reverse direction in the sensor so that the loadis restored to the original position. Since the quantity of the feedbackcorresponds to the magnitude of the acceleration, the acceleration isobtained from the quantity of the feedback. According to this method,the displacement is substantially constant regardless of theacceleration.

Similarly to the above-described embodiment, the displacement is oftenmeasured by the electrostatic capacity method or the piezoelectricresistance method. Furthermore, force is applied to the load inaccordance with the quantity of the feedback by using an electrostaticforce or a magnetic force.

In the servo type sensor, the relationship between the final outputsignal and the acceleration is usually expressed by Equation (II). It isassumed that a second force F is applied to the load in addition to theforce which corresponds to the quantity of the feedback in the servosystem. The output from the sensor at this time can be expressed by thefollowing equation:

    V(α, F)=p(α+F/m)+q . . .                       (VII)

In the case where forces F₁ and F₂ of predetermined magnitudes areapplied, the following relationships are held: ##EQU9## Substracting thelower equation from the upper equation, the following equation can beobtained:

    V(α,F.sub.1)-V(α, F.sub.2)=p(F.sub.1 -F.sub.2)/m . . . (IX)

If V(α, F₁), V(α, F₂), F₁, F₂ and m are known, p can be obtained.

When the sensor device is brought into upside down by an actuator suchas a motor with the application of the force F (F may be 0) maintained,the direction of each of the acceleration and the second force appliedto the sensor is inverted. Therefore, the output becomes as follows:

    V(-α,-F)=p(-α-F/m)+q . . .                     (X)

Adding Equations (7) and (10), the following relationship is obtained:

    V(α,F)+V(-α,-F)=2q . . .                       (XI)

Therefore, q can be obtained from Equation (XI).

FIGS. 12 and 13 respectively illustrate specific examples of thestructures of the sensor devices. FIG. 12 illustrates the structure formeasuring a displacement by using the electrostatic capacity, while FIG.13 illustrates the structure for measuring a displacement by using thepiezoelectric resistance device. In each of the two structures, theforce according to the quantity of the feedback and the second force areapplied to the load by using the electrostatic force for the purpose offorming a servo system. Reference numerals 61a and 61b representelectrodes for detecting the electrostatic capacity which corresponds tothe displacement. Reference numerals 62a and 62b represent electrodesfor applying the electrostatic force for the servo effect. Referencenumerals 63a and 63b represent electrodes for applying the electrostaticforce for the purpose of applying the second force to the load. Theseelectrodes can be used in a combined manner by arranging a circuit forthe servo system and that for applying the electrostatic force to be ofproper structures.

Since the displacement of the load is substantially constant in theservo type sensor, the gap between each of the two electrostatic forceapplying electrodes 63a and 63b and the load is constant. Therefore, theconstant forces F₁ and F₂ shown in Equation (VIII) can be obtainedsimply by varying the level of the voltage to be applied to theelectrostatic force applying electrodes 63a and 63b. Therefore, themagnitude of each of the forces F₁ and F₂ can be calculated if the areasof the electrodes, the sizes of the gaps and the level of the voltage tobe applied are previously known.

According to the above-described embodiment, an advantage can beobtained that the change in the acceleration output characteristics ofthe servo type acceleration sensor as the time proceeds can becorrected.

Then, an embodiment in which the sensor having a self-diagnosis functionis applied to an air-fuel ratio sensor for an automobile will bedescribed below. FIG. 14 illustrates the relationship between excess airfactor λ, the exhaust gas density and the electromotive force. As iswell known, the residual oxygen density is increased with the excess airfactor λ in the lean region (λ>1), while the density of unburnt gasessuch as carbon monoxide and hydrogen is increased in the rich region(λ<1) with the decrease in the excess air factor λ. Hitherto, an O₂sensor utilizing electromotive force eλ showing step-wise outputcharacteristics around the stoichio-metrical air-fuel ratio (λ=1) hasbeen employed as the key sensor for controlling the engine in order tosatisfy the automobile emission gas regulation. However, the O₂ sensorcannot satisfactorily meet the desire of enlarging the output in therich region and improving the purification of the exhaust gases at thestoichiometrical air-fuel ratio and the economical efficiency in thelean region since the O₂ sensor is able to detect only thestoichio-metrical air-fuel ratio. Therefore, there has been a demand foran air-fuel ratio sensor capable of continuously and accuratelydetecting the excess air factor λ over a wide range from the rich regionto the lean region in order to achieve a most suitable combustioncontrol of the engine. Accordingly, there has been known an air-fuelratio sensor which utilizes the diffusion controlling phenomenon of theabove-described various gas components in a gas diffusion film and theoxygen pump phenomenon of a zirconia solid electrolyte.

FIG. 15 illustrates an example of the V-I characteristics of an air fuelratio sensor of the type described above, wherein the relationshipbetween exciting voltage E applied to a detection portion and pumpelectric current I_(P) passing through the detection portion is shown.As shown in this drawing, the pump electric current I_(P) exhibits apredetermined value in a certain range of exciting voltage. Thepredetermined value is a value determined by diffusion resistance R inthe gas diffusion film and the excess air factor λ, the value beingcalled a "critical current value". The excess air factor λ is measuredfrom the level of the critical current value I_(P).

The diffusion resistance R varies depending upon the adhesion of dust tothe gas diffusion film and microcracks formed in the gas diffusion film,causing the critical current value I_(P) to be varied correspondingly.In the case of the adhesion of dust to the gas diffusion film, thecritical current value is reduced since the diffusion resistance isincreased. On the other hand, the diffusion resistance is reduced in thecase of the microcracks, causing the critical current value to beincreased. In either of the above-described two cases, the excess airfactor λ cannot be detected accurately.

FIG. 16 illustrates the characteristics obtained by converting thecritical current value which corresponds to the excess air factor λ intothe output voltage V_(O). The initial characteristics of the air-fuelratio sensor is shown in the drawing by a continuous or solid line. Theoutput characteristics obtained when the diffusion resistance R of thegas diffusion apertures has been increased as the time proceeds aredesignated by a one-dot line, while the output characteristics obtainedwhen the diffusion resistance R has been decreased are designated by atwo-dot line. As shown in the drawing, the output voltages at the zeropoint of the air-fuel ratio sensor, that is, at the stoichiometricalair-fuel ratio point (λ=1), are not varied. The reason for this lies inthat the critical current value is, as shown in FIG. 15, zero at thestoichiometrical air-fuel ratio point. The output voltages are variedonly in the rich region (λ<1) and the lean region (λ>1), that is, onlythe sensitivity of the air-fuel ratio sensor is changed.

Then, an air-fuel ratio sensor having a self-diagnosis function will bedescribed with reference to FIG. 17, the air-fuel ratio sensor beingdesigned such that the amount of the change in the characteristicvariable of the sensor caused when the calibrating electric signal isapplied, is measured and the change in the sensitivity as the timeproceeds is corrected in accordance with the electric signal and theamount of the change in the characteristic variable of the sensor.

Referring to FIG. 17, the detection portion of the air-fuel ratio sensorcomprises a zirconia solid electrolyte 100, porous electrodes 101 and102 and a gas diffusion film 103. The zirconia solid electrolyte 100 isof a tubular shape with the porous electrode 101 formed on the innersurface thereof and the porous electrode 102 and the gas diffusion film103 formed on the outer surface thereof. The porous electrode 101 isexposed to the atmosphere while the porous electrode 102 and the gasdiffusion film 103 are exposed to the exhaust gases with the zirconiasolid electrolyte 100 acting as a partition wall.

The detection means comprises a switch 104 and a portion 105 formeasuring critical current I_(P), while the stimulating means comprisesa portion 106 for supplying a predetermined current I_(P) *. Aprocessing means 107 has the self-calibration function and preferablycomprises a microcomputer. FIG. 17 schematically illustrates that theswitch 104 comprises contacts 108, 109 and 110 and is arranged such thatthe portion 106 for supplying the predetermined current I_(P) * isoperated when a connection between the contact 108 and the contact 109is established and such that, when a connection between the contact 108and the contact 110 is established, the portion 105 for measuring thecritical current I_(P) is operated. When no connection is establishedbetween the contact 108 and any of the contacts 109 and 110, both theportion 105 for measuring the critical current I_(P) and the portion 106for supplying the predetermined current I_(P) * are not operated.

The density of the gas contained in the exhaust gases is changed inaccordance with the excess air factor λ. Therefore, the critical currentI_(P) which corresponds to the content of the unburnt gas such as theresidual oxygen and the carbon monoxide is measured by the portion 105for measuring the critical current I_(P) when the contacts 108 and the110 are connected. The output characteristics of the air-fuel ratiosensor is automatically diagnosed periodically (for example, about everymonth). That is, if the excess air factor λ continuously shows apredetermined value (preferably the stoichio-metrical air-fuel ratioλ=1) for a considerably long time, the self diagnosis function portion107 controls the switch 104 so as to establish a connection between thecontacts 108 and 109. As a result, the portion 106 for supplying thepredetermined current I_(P) * is operated. The portion 106 for supplyingthe predetermined current I_(P) * forcibly supplies the calibratingelectric signal I_(P) * to the detection portion. As a result, oxygen ofa predetermined quantity, which corresponds to the predetermined currentI_(P) *, can be supplied from the porous electrode 101, which is exposedto the atmosphere, through the zirconia solid electrolyte 100 to theporous electrode 102, which is exposed to the exhaust gas.

The oxygen thus supplied is discharged into the exhaust gas from theporous electrode 102 through the gas diffusion film 103. Since the rateof discharge of the oxygen is determined depending upon the diffusionresistance R of the gas diffusion film 103, the change in thesensitivity of the air-fuel ratio sensor as the time proceeds can bediagnosed by measuring the change in the content of the oxygen at theinterface between the porous electrode 102 and the gas diffusion portion103. In the case where the diffusion resistance R of the gas diffusionfilm 103 is large, the rate of discharge of the oxygen is low, while thedischarge rate is high when the diffusion resistance R is small.

As described herein above, the fact that the oxygen discharge rate islowered than the initial one is an evidence that the sensitivity of theair-fuel ratio sensor has been deteriorated. On the contrary, the factthe rate is raised is an evidence that the sensitivity has beenincreased. Immediately after the portion 106 for supplying thepredetermined current I_(P) * has been operated, the portion 105 formeasuring the critical current I_(P) is intermittently operated inresponse to a command issued from the self-diagnosis function portion107. As a result, the change in the oxygen discharge rate as the timeproceeds (that is, the change in the diffusion resistance R as the timeproceeds) is estimated. If the change in the diffusion resistance R asthe time proceeds is known, an accurate output voltage V_(out)corresponding to the excess air factor λ can be obtained by correctingthe sensitivity of the air-fuel ratio sensor by the self-diagnosisfunction portion 107.

Then, a method of diagnosing the change in the diffusion resistance R asthe time proceeds will be described in detail with reference to FIG. 18.Graph (a) illustrates the output characteristics of the portion 105 formeasuring the critical current I_(P), (b) illustrates the state of theoperation of the portion 105 for measuring the critical current I_(P),(c) illustrates the state of the operation of the portion 106 forsupplying the predetermined current I_(P) * and (d) illustrates a statein which the contact 108 is fully opened.

As shown in graph (a), the excess air factor λ is controlled from λ',through λ(=1) to λ" in order to realize a proper air-fuel ratio whichcorresponds to the state in which the automobile is operated. After theoperation at the stoichiometrical air-fuel ratio (λ=1) has beencontinued for a certain time period, the connection of the contact 108in the switch 104 is changed from the contact 110 to the contact 109. Asa result, the portion 106 for supplying the predetermined currentI_(P) * is operated for a certain time (to) so that oxygen of apredetermined quantity is forcibly supplied from the porous electrode101 to the porous electrode 102. Then, the portion 105 for measuring thecritical current I_(P) is intermittently operated so as to detect thechange in the critical current I_(P) caused in the detection portion.The current I_(P) detected is gradually decreased as shown by a one-dotline in graph (a). It takes a time τ for the current I_(P) to be loweredto a predetermined level I.sub. PC. The reason for the decrease in thelevel of the current I_(P) lies in that oxygen at the interface betweenthe porous electrode 102 and the gas diffusion film 103 is dischargedinto the exhaust gas, causing the density of the oxygen at the interfaceto be lowered gradually.

When the diffusion resistance R of the gas diffusion film 103 isreduced, the above-described time τ becomes shorter than the initialvalue τO. On the contrary, when the diffusion resistance has beenincreased, the time τ becomes larger than the initial value τO.Therefore, the self-diagnosis function portion 107 performs theswitching correction in such a manner that the sensitivity of theair-fuel ratio sensor is decreased in the former case, while the same isimproved in the latter case. As a result, an accurate output voltageV_(OUT) can always be obtained.

As described above, the change in the sensitivity as the time proceedscan be corrected in accordance with the amount of the change in thecharacteristic variable (I_(P)) of the air-fuel ratio sensor which iscaused due to the calibrating electric signal (I_(P) *) applied.

According to the present invention, the calibration operation can beperformed in addition to the measuring operation. Therefore, an accurateoutput can be detected continuously.

What is claimed is:
 1. A detector system for detecting a physicalquantity as a quantity of electricity, comprising:detection means fordetecting the physical quantity to generate an output; means forproviding said detection means with a stimulating signal based upon aself-calibration signal for self-calibration of input/outputcharacteristics of said detection means; and signal processing meansoperative to supply said self-calibration signal to said stimulatingmeans and to compare said output of said detection means based upon saidstimulating signal with predetermined input/output characteristics tothereby correct said output of said detection means on the basis of theresults of the comparison.
 2. A detector system according to claim 1,further including a communication device, and means for displaying theresults of said self-calibration and said characteristics correction,said display means being provided for one of said processing means andsaid communication device.
 3. A detector system according to claim 1,wherein said detection means, said stimulating means and said processingmeans are formed on a silicone substrate.
 4. A detector system accordingto claim 3, wherein said detection means and said stimulating means areformed on a silicone. monocrystal substrate and are fabricated byanisotropic etching with a mask formed by an oxide film or a nitridefilm formed on said silicone monocrystal substrate.
 5. A detector systemaccording to claim 1, wherein said detection means and said stimulatingmeans are electrostatically coupled via borosilicate glass.
 6. Adetector system according to claim 1, wherein said detection meanscomprises a strain generation body including a piezoelectric device, acalibrating voltage signal being applied via an electrostatic capacityformed at the central portion of said strain generation body as saidstimulating means, the resistance of said piezo-electric resistancedevice being measured and the sensitivity being calibrated in accordancewith a change in said resistance.
 7. A detector system according toclaim 1, wherein said detection means is a variable electrostaticcapacity comprising a supported movable electrode and a fixed electrode,a calibrating voltage signal is applied via an electrostatic capacityserving as said stimulating means and formed by said movable electrodeand another fixed electrode so as to measure the capacity of saidmovable electrostatic capacity, and the sensitivity is calibrated inaccordance with a change in said capacity.
 8. A detector systemaccording to claim 1, wherein said detection means comprises electrodesformed on opposite sides of a fixed electrolyte, a gas diffusion filmand a critical current measuring portion, said stimulating meanscomprising means for supplying a predetermined current so as to apply acalibrating current signal, a damping time of said critical currentbeing measured so that the sensitivity is calibrated in accordance witha change in the damping time.